In response to recent increasing demands for the capability of data communication in mobile phone systems, a growing number of card data communication terminals are being used.
While known mobile phone terminals are supplied with power from built-in batteries for operation, PC card (PCMCIA card) terminals are typically supplied with power by host devices, such as personal computers, because these PC card terminals do not have batteries therein due to, for example, a size restriction.
Card slots for PC cards are typically provided in portable terminals such as so-called notebook PCs and PDAs (personal information assistants). Electrical characteristics such as signals and power supply and mechanical characteristics such as size in these card slots are specified as standards (as in the PC Card Standard by the standards organizations PCMCIA in the USA and JEITA). PC card terminals also comply with these standards. The standard for power supply specifies 3.3 V±0.3 V and a maximum of 1 A for 32-bit cards (Card Bus), for example.
In wireless communication techniques supporting recent high data communication speeds and cellular systems, unlike wireless LANs, assumed to be used in wide areas (e.g., CDMA2000 and W-CDMA), baseband processors such as high-speed CPUs and maximum transmission powers of 20 dBm (100 mW) or more are required, which leads to more power consumption. For example, in device types that employ the CDMA2000 1× technique and are rated a maximum transmission power of 23 dBm, even PC card terminals that do not have, for example, LCDs (liquid crystal displays) or backlight may exhibit a consumption current of about 1 A at the time of maximum output transmission.
For communication techniques in which terminal transmission power is controlled by a base station, such as the CDMA2000 1× technique, terminals perform transmission with the minimum required transmission power. More specifically, radio waves from a terminal located near a base station easily reach the base station, and therefore, such a terminal performs transmission (communication) with a low output. On the other hand, a terminal located distant from a base station performs transmission (communication) with a high output, which causes the current consumption to increase.
In view of these circumstances, a communication technique for reducing power consumption of mobile communication terminals is proposed, as described in, for example, Patent Document 1. In this technique, when a mobile communication terminal detects a drop in supply voltage, the network is informed of the fact, and a value smaller than normal is determined anew as the maximum transmission power required to continue communication.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2003-309516